Rickenbacker louvered air return
If you have a wall duct opening that is located in between two baseboards then this is a good option. Can be made in any dimension, including fractional sizes. Red oak and paint grade maple are the standard woods, and other woods are available with an upcharge.
These are ordered by the overall dimensions, not the duct opening size. Pricing is the overall length times the overall width times .64 in the event you have 100 square inches or more. Below 100 square inches the price is a flat fee of $63.95. Shipping is 10%, but at least $12.95